A number of negative regulators of immune responses, cellular proliferation and hematopoiesis have been described. Protein or peptide negative regulators include transforming growth factor-beta (1,2), leukemia inhibitory factor (3), macrophage inflammatory protein-1-alpha (4,5), Ac-Ser-Asp-Lys-Pro (6), and pGlu-Glu-Asp-Cys-Lys (7). Lipid and lipid-related negative regulators include the prostaglandins (8), the leukotrienes (9), and the glycosphingolipids (10).
Preparations derived from bone marrow cells and showing immunosuppressive activity have also been described (11 and PCT application number WO91/19506).
The new factor described herein is distinguishable from all of these previously described factors both by chemical properties and by its biological activity, as will be more fully described below.